This blog discusses current weather, weather prediction, climate issues, and other topics

Saturday, December 3, 2011

The Irony of Wintertime High Pressure

High pressure is good, right? Sunny skies, light winds, and dry conditions? A nice break from stormy, wet conditions? Not necessarily true here over the western lowlands during cool season. For us, high pressure often brings something I personally dislike more than storms---endless low clouds. And often air pollution as well.

It is Saturday morning and here is the latest visible satellite picture. Low clouds over the Willamette Valley and western Washington lowlands. There are clear skies over the mountains, along much of the coast, and yes over those fortunate folks in Sequim and Victoria. Some higher clouds are found over eastern Washington from a weak system moving southward out of Canada.

As shown by this morning's surface pressure chart. high pressure dominates our region, with the center of the high pressure offshore. No storms, no rain, no snow. The pressure gradients over us are weak and the winds are light. It appears that we are going to be stuck in this pattern for the next week. Lucky us.

The problem is that in the Northwest, high pressure around during the winter is a low cloud production machine. First, with a high over us or nearby the winds are light, since the large pressure changes (gradients) are at the periphery of highs. Less winds means less turbulent mixing of air in the vertical. Highs generally bring sinking air in the troposphere and thus lack of middle and higher layer clouds. Lack of clouds means the earth can effectively radiate infrared radiation to space and thus cool the surface. The cooling surface cools the very lowest portions of the atmosphere. Air can cool to saturation and give you fog and low clouds.

Saturday morning in Seattle. Mt. Rainier is straight ahead. See it?

And having cool air near the surface and warmer air above leads to an inversion--temperatures increasing with height. And inversions cause the atmosphere to become very stable, with little mixing in the vertical (think of oil above water). No mixing means you can't bring the warmer air aloft down to the surface...and one stays cool and murky. Here are the latest temperatures at Seattle--you can see the inversion from 200 to 500 meters above the surface (immediately above the surface there is an issue of a different temperature sensor)

And did I mention that the very long nights this time of the year gives the surface a lot of time to cool and the weak sun hardly produces enough warming to heat the surface and mix this out? The same weather pattern in the summer would be no problem...we would have sun after perhaps some morning clouds.

And it gets worse. Since we have had rain for a while the surface is moist, with lots of water to give to the atmosphere, and with high pressure offshore, moisture streams in at low levels off the ocean. And once in place fog and low clouds cool at the top (infrared cooling to space) and tend to reinforce themselves. Diabolical.

Now today and tomorrow, the inversion will be weak and shallow enough for there to be a good chance it will mix out a bit this afternoon and give us some sun. Good time to rake leaves or take a walk. But as we get into next week, we will probably experience days in which we stay in the murk all day. The good news is that you can get out of this by going up...head to the mountains or even a high park--such as Cougar Mountain regional park. Or take a walk up Tiger Mountain (top 2500ft). The worst place to be is the southern Sound lowlands.

Finally, a major issue with such situations is air quality. Inversions and light winds bring deteriorating conditions and in fact the Puget Sound Clean Air Agency has a burn ban going on right now. Just to prove to you that air quality is declining, here are some plots from two local reporting stations. We are now seeing moderate AQ conditions and it won't be long before we are in unhealthy ones.

Anyway, it helps to know what is going on and that escape is always possible with a short drive. It gives one a sense of control.

Really Nice Graphs! Can you say that the Inversion chart actually depicts a Double Inversion? Up in Bonney Lake above Tacoma, we're seeing the doughnut hole effect with fog and clouds at night covering the Plateau, but at the same time clear skies and stars straight above!

Thank you for the insights Cliff. I know that you do not have a big aversion to putting yourself out on a limb with early predictions that may or may not pan out and I appreciate that. There is as much to be learned from mistakes as success IMO. So with that encouragement, any speculation as to why, apparently more so in recent years, weather systems seem to be growing in size? This record high pressure system, last years swallow your gum low, "Frankinstorm",(I hate that name), the huge floods in Australia, the Russian heat wave last year, the Texas, off the charts drought, Pakistani floods, last winter's eastern snow fall, the list goes on. Can the loss of the ~2.7 million sq. km of ice in the Arctic be a contributing factor?

We've been in the gunk all day in the islands. Seems a little clearing out towards Friday Harbor. This pattern seems so persistent I wonder if we are going to have the driest December on record. Too soon to tell, but a good start.

I grew up in the S.F. Bay Area and I found that one could sometimes escape the inland winter low clouds by heading to the ocean. Any truth to that in this region? I note on the satellite photo in your blog post that significant sections of the SW WA coast appear to be clear. I've lived here more than 20 years now but I can't say I've kept an eye out for that correlation, if it exists.

Sorry for all the comments, but I did a little sleuthing. I'm guessing a very weak shortwave is responsible for the enhanced precipitation tonight. It looks like another one may brush us around Tuesday and a slightly stronger one on Thursday. If these prove strong enough it may not be all gloom and doom this week - I hope.

KC. An interesting observation and I will attempt an insight. (subject to Cliff's correction if need be of course.) Earth cools faster than water and also these high pressure fog/cloud reactions tend to have a limited top. Recall the inversion temperature map. As the air warms the clouds/fog evaporates and skies clear. Walk up a mountain and you can emerge from gloom to sun in a surprisingly short distance, during still air especially. The lake is warming the air above just enough that you emerge a bit sooner at that point.

Leif, Thanks for your Insight too. Also,I agree this isn't boring, at least not up on the Plateau; the sun makes it seem warm, and just in the morning too! I'm willing to bet any amount of money that this high pressure is the one they indicate as being the shifting point in the El Nino/La Nina Pattern. So we should be getting the La Nina effects coming soon, I think!

It's also a departure from normal. Anyone else feel that unease creeping up their back? That despite La Nina, it's going to be another wimpy winter, that is until late March when it cools down again too late for snow, but just in time to usher in another terrible, cool Spring/Summer. Bring back the rain please...

http://www.seattlepi.com/local/komo/article/Trooper-flips-cruiser-in-fog-black-ice-2345423.php Will he get an expensive "driving too fast for conditions ticket"? A civilian driver would if the same trooper happened upon that scene.

KC...I hope you're right and we start seeing the effects of La Nina soon! Rumor has it that the week before Christmas it's supposed to cool down and get more moisture.....perhaps snow? We can only hope! That would be soooo awsome to have a white Christmas! But, alas, we can't trust the forcast that far out so we'll just have to wait with our fingers crossed! And, I guess I can't complain too much about the winter being boring, since technically it isn't even winter until December 21st (or there-a-bout)..... ;)

Cliff, I notice that the Puget Sound Air Pollution Control Agency seems to have posted a burn ban for Pierce and Snohomish, but NOT for King County! I live in Snohomish, and with all this chilly weather, I would like to use my woodstove, but can't. Why don't they post a ban in King County first? After all, King has more cars, more fireplaces, more factories, and they also are right in the center of the Puget valley. How could it be that we more rural folks get more pollution... It doesn't make sense to me. Or are there some shenanigans going on with PSACA?

Similar conditions to those you describe in SF occur here along the coast. Case in point my family spent this past weekend at our cabin at Iron Springs and the weather was sunny both days, with an especially magnificent sundown Sunday afternoon.

As with the Bay area, the WA coast is frequently plagued by strong winds and fog during the summer months and delightful weather in April/May and September/October.

My advice to friends hoping to find nice weather at the beach is to come when breezes blow offshore; that is unless of course, you're pining for a big blow and wild surf.